1. Field of the Invention
The present invention relates generally to word synchronization for servo read signals in tape storage systems. More particularly, methods and apparatus are provided for detecting sync words in pulse-position-modulation (PPM) encoded servo pattern read signals read from magnetic tape in tape drives.
2. Description of Related Art
In general, synchronous digital communication receivers include various types of synchronization subsystems such symbol synchronization and word synchronization (also known as frame synchronization) subsystems. Assuming symbol synchronization has already been achieved, the task of word synchronization consists of determining the position of the sync word within the received data stream.
Various techniques for word synchronization are described in “Frame synchronization techniques,” R. A. Scholtz, IEEE Trans. on Communications, vol. 28, August 1980, pp. 1204-1213. In early work on word synchronization, the concept of the correlation decision rule was introduced (see R. H. Barker, “Group synchronization of binary digital systems,” in Communication Theory, W. Jackson, Ed. London. Butterworth, 1953, pp. 273-287). According to this rule, bits in the received data stream are correlated with respective sync word bits and a sync word detected at the position of maximum correlation.
U.S. Pat. No. 5,073,906 employs a correlation technique for soft output samples in a mobile satellite communications receiver wherein samples are correlated with the sync word bits to produce a cross-correlation value which is compared with a threshold dependent on mean power for sync word detection.
An optimum word synchronization algorithm specifically for antipodal binary signals that are received in the presence of additive white Gaussian noise was proposed in “Optimum frame synchronization,” J. L. Massey, IEEE Trans. on Commun., vol. 20, April 1972, pp. 115-119, (see also the Scholtz reference above). Specifically, the correlation decision rule for the optimum word synchronization algorithm for antipodal binary signals turns out to be the correlation decision rule modified by an additive correction term that depends on the signal-to-noise ratio (SNR). High-SNR and low-SNR approximations of the optimum word synchronization algorithm were also presented. Simulation results have demonstrated that the high-SNR approximation of the optimum word synchronization algorithm for antipodal binary signals performs almost as well as the optimum word synchronization algorithm (see “Some optimum and suboptimum frame synchronizers for binary data in Gaussian noise,” P. T. Nielsen, IEEE Trans. on Communications, vol. 21, June 1973, pp. 770-772).
In “Frame Synchronization for Gaussian channels,” G. L. Lui and H. H. Tan, IEEE Trans. on Communications, vol. 35, August 1987, pp. 818-829, the optimum word synchronization algorithm for antipodal binary signals in vector space was extended to both coherent and non-coherent phase demodulation on Gaussian channels assuming a maximum-likelihood one-shot receiver structure (see “Principles of Communication Engineering”, J. M. Wozencraft and I. M. Jacobs, John Wiley & Sons, 1965).
In tape storage systems, timing-based servo (TBS) technology is employed for recording servo information in dedicated servo tracks extending longitudinally of the tape adjacent the data tracks. The LTO (Linear Tape Open) consortium adopted the TBS approach and standardized a robust, scalable, dedicated servo pattern for LTO tape drives which provides backward compatibility and remains unchanged in the follow-on LTO standards with higher track density. The TBS servo pattern is detailed in Standard ECMA-319, “Data interchange on 12.7 mm 384-track magnetic tape cartridges—Ultrium-1 format,” June 2001, pp. 48 to 56.
The servo pattern consists of magnetic transitions defining a series of stripes with two different azimuthal slopes. The transverse position of the head can be derived from the relative timing of pulses generated by a narrow servo head reading the stripe pattern. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting generation of the transverse position error signal (PES). The LPOS information is encoded by shifting transitions (stripes) in the servo pattern from their nominal pattern position in the longitudinal direction of the tape. LPOS information is recorded in 36-bit words over 36 frames of the servo pattern, each LPOS word indicating a specific, absolute longitudinal position on the tape. Each servo frame thus encodes one bit of LPOS information, the bit value being indicated by the particular shift in the servo pattern stripes.
The first 8 bits of a 36-bit LPOS word constitute a known synchronization word. Since the LPOS bits are recorded by shifting the longitudinal position of servo frame stripes, the resulting servo head read signal is a PPM-encoded signal in which the sync words must be identified for recovery of the position data.
Conventional synchronization systems in LTO drives apply the correlation decision rule to hard output bits derived from the PPM-encoded servo pattern read signal. Hard output bits are detected by a matched filter following interpolation of servo read signal samples as detailed in our U.S. Pat. No. 7,245,450 and “Synchronous Servo Channel Design for Tape Drive Systems,” Cherubini et al., Proc. 17th Annual ASME Information Storage and Processing Systems Conf. ISPS 2007, Santa Clara University, CA, Jun. 18-19, 2007, pp. 160-162.
A sync word detector employed in conventional LTO tape drives is illustrated in FIG. 1 of the accompanying drawings. The stream of hard output bits {circumflex over (b)}k, of value +1 or −1, is clocked through a series of seven delay elements D whose inputs/outputs supply a sliding block of eight bits {circumflex over (b)}k, {circumflex over (b)}k−1, . . . , {circumflex over (b)}k−7 to respective multipliers.
The known sync word bits p0, p1, . . . , p7 (where p0=1 and pi=−1 for i=1, . . . , 7) are supplied to the other inputs of the multipliers as shown. The resulting bit correlation values are summed to produce a block correlation value Ck at each position of the sliding 8-bit block in the output bit-stream. The block correlation values are supplied to a detector which outputs a word sync indicator when Ck=8, indicating a match between the current block and the expected sync word.
The robustness of the word synchronization operation in the presence of disturbances and noise is important for reliable operation of tape drives. Errors in reading back LPOS information and in the synchronization of LPOS words can generate false position information during normal drive servo operation. An improved word sync system for tape drives would therefore be desirable.